The present invention relates to the production of paucilamellar lipid vesicles. More particularly, the present invention relates to a method of producing paucilamellar lipid vesicles having an aqueous or organic amorphous liquid or solid central cavity. These paucilamellar lipid vesicles have a wide variety of non-phospholipid surfactants as their primary structural material, with a small amount of phospholipid added if desired.
Lipid vesicles are substantially spherical structures made of materials having a high lipid content, e.g., surfactants or phospholipids. The lipids of these spherical vesicles are organized in the form of lipid bilayers. The lipid bilayers encapsulate an aqueous volume which is either interspersed between multiple onion-like shells of lipid bilayers (forming multilamellar lipid vesicles or "MLV") or the aqueous volume is contained within an amorphous central cavity. The most commonly known lipid vesicles having an amorphous central cavity filled with aqueous medium are the unilamellar lipid vesicles. Large unilamellar vesicles ("LUV") generally have a diameter greater than about 1.mu. while small unilamellar lipid vesicles ("SUV") generally have a diameter of less than 0.2.mu.. There are a variety of uses for lipid vesicles including the use as adjuvants or as carriers for a wide variety of materials.
Although substantially all the investigation of lipid vesicles in recent years has centered on multilamellar and the two types of unilamellar lipid vesicles, a fourth type of lipid vesicle, the paucilamellar lipid vesicle ("PLV"), exists. This lipid vesicle has barely been studied heretofore and has only been manufactured previously with phospholipids. PLV's consist of about 2 to 8 peripheral bilayers surrounding a large, unstructured central cavity. In all the previously described PLV's, this central cavity was filled with an aqueous solution. See Callo and McGrath, Cryobiology 1985, 22(3), pp. 251-267.
Each type of lipid vesicle appears to have certain uses for which it is best adapted. For example, MLV's have a higher lipid content then any of the other lipid vesicles so to the extent that a lipid vesicle can encapsulate or carry a lipophilic material in the bilayers without degradation, MLV's have been deemed the most advantageous for carrying lipophilic materials. In contrast, the amount of water encapsulated in the aqueous shells between the lipid bilayers of the MLV's is much smaller than the water which can be encapsulated in the central cavity of LUV's, so LUV's have been considered advantageous in transport of aqueous material. However, LUV's, because of their single lipid bilayer structure, are not as physically durable as MLV's and are more subject to enzymatic degradation. SUV's have neither the lipid or aqueous volumes of the MLV's or LUV's but because of their small size have easiest access to cells in tissues.
PLV's, which can be considered a sub-class of the MLV's, are a hybrid having features of both MLV's and LUV's. PLV's appear to have advantages as transport vehicles for many uses as compared with the other types of lipid vesicles. In particular, because of the large unstructured central cavity, PLV's are easily adaptable for transport of large quantities of aqueous-based materials. However, the multiple lipid bilayers of the PLV's provides PLV's with the capacity to transport a greater amount of lipophilic material in their bilayers as well as with additional physical strength and resistance to degradation as compared with the single lipid bilayer of the LUV's. As illustrated in the present application, the aqueous cavity of the PLV's can be filled wholly or in part with an apolar oil or wax and then can be used as a vehicle for the transport or storage of hydrophobic materials. The amount of hydrophobic material which can be transported by the PLV's with an apolar core is much greater than can be transported by MLV's.
Conventional multilamellar lipid vesicle producing methods start by dissolving the lipids, together with any lipophilic additives, in an organic solvent. The organic solvent is then removed by evaporation using heat or by passing a stream of an inert gas (e.g., nitrogen) over the dissolved lipids. The residue is then hydrated with an aqueous phase, generally containing electrolytes and additives such as hydrophilic biologically-active materials, to form large multilamellar lipid membrane structures. In some variations, different types of particulate matter or structures have been used during the evaporation process to assist in the formation of the lipid residue. Those in the field have shown that by changing the physical structure of the lipid residue, better vesicles form upon hydration. Two recent review publications, Gregoriadis, G., ed. Liposome Technology (CRC, Boca Raton, Fla.), Vols. 1-3 (1984), and Dousset and Douste-Blazy (in Les Liposomes, Puisieux and Delattre, Editors, Techniques et Documentation Lavoisier, Paris, pp. 41-73 (1985)), summarize the methods which have been used to make MLV's. Phospholipid PLV's have been made by minor modifications of these processes.
No matter how the MLV's or PLV's are formed, once made it is necessary to determine the effectiveness of the process. Two measurements commonly used to determine the effectiveness of encapsulation of materials in lipid vesicles are the encapsulated mass and captured volume. The encapsulated mass is the mass of the substance encapsulated per unit mass of the lipid and is often given as a percentage. The captured volume is defined as the amount of the aqueous phase trapped inside the vesicle divided by the amount of lipid in the vesicle structure, normally given in ml liquid/g lipid.
All of the early lipid vesicle or liposome studies used phospholipids as the lipid source for the bilayers. The reason for this choice was that phospholipids are the principal structural components of natural membranes. However, there are many problems using phospholipids as artificial membranes. First, isolated phospholipids are subject to degradation by a large variety of enzymes. Second, the most easily available phospholipids are those from natural sources, e.g., egg yolk lecithin, which contain polyunsaturated acyl chains that are subject to autocatalyzed peroxidation. When peroxidation occurs, the lipid structure breaks down, causing premature release of encapsulated materials and the formation of toxic peroxidation byproducts. This problem can be avoided by hydrogenation but hydrogenation is an expensive process, thereby raising the cost of the starting materials. Cost is a third problem associated with the use of phospholipids on a large scale. A kilogram of egg yolk lecithin pure enough for pharmacological liposome production presently costs in excess of $1,000. This is much to high a cost for a starting material for most applications.
Recently, there has been some indication, particularly from the L'Oreal group, that commercially available surfactants might be used to form the lipid bilayer in liposome-like multilamellar lipid vesicles. Both surfactants and phospholipids are amphiphiles, having at least one lipophilic acyl or alkyl group attached to a hydrophilic head group. The head groups are attached to one or more lipophilic chains by ester or ether linkages. Commercially available surfactants include the BRIJ family of polyoxyethylene acyl ethers, the SPAN sorbitan alkyl esters, and the TWEEN polyoxyethylene sorbitan fatty acid esters, all available from ICI Americas, Inc. of Wilmington, Del.
The methods and materials disclosed herein for producing the paucilamellar lipid vesicles all yield vesicles with a high aqueous or oil volume. Electron micrographs confirm that the paucilamellar lipid vesicles are distinct from the LUV's and the classic MLV's.
Accordingly, an object of the invention is to provide paucilamellar lipid vesicles from non-phospholipid materials.
Another object of the invention is to provide a method of producing paucilamellar lipid vesicles which is rapid and uses relatively inexpensive materials.
A further object of the invention is to provide a method of manufacturing vesicles having a membrane-like outer structure surrounding an oil or fat droplet.
An additional object of the invention is to provide a vehicle for transport of oil soluble materials.
These and other objects and features of the invention will be apparent from the detailed description and the claims.